Hierarchical encoding methods with spatial scalability are known. Scalability represents the ability to stagger information to make it decodable at multiple resolution and/or quality levels. More specifically, a data stream generated by this type of encoding method is divided into several layers, in particular a base layer and one or more enhancement layers. These methods are used in particular to adapt a single data stream to variable transport conditions (bandwidth, error ratios, etc.), and to the expectations of the customers and the varying capabilities of their receivers (CPU, specifications of the display device, etc.). In the particular case of spatial scalability, the part of the data stream corresponding to low resolution pictures of the sequence can be decoded independently of the part of the data stream corresponding to high resolution pictures. On the other hand, the part of the data stream corresponding to the high resolution pictures of the sequence can be decoded only from the part of the data stream corresponding to the low resolution pictures.
Hierarchical encoding with spatial scalability makes it possible to encode a first data part called the base layer, relative to the low resolution pictures and, from this base layer, a second data part called the enhancement layer, relative to the high resolution pictures. Normally, each macroblock of the high resolution picture is temporally predicted according to a conventional prediction mode (for example, bidirectional prediction mode, direct prediction method, early prediction mode, etc.) or is predicted according to an inter-layer prediction method. In this latter case, motion data (for example, a partitioning of the macroblock into blocks, possibly motion vectors and reference picture indices) and where appropriate texture data associated with a block of pixels of the high resolution picture, is deduced or inherited from the motion data or respectively texture data, is associated with blocks of pixels of a low resolution picture. However, the known methods do not allow such predictors to be generated in the case where the low resolution sequence and the high resolution sequence are interlaced.